1. Combinatorial design of simplified high-performance chiral phase-transfer catalysts for practical asymmetric synthesis of alpha-alkyl- and alpha,alpha-dialkyl-alpha-amino acids
Masanori Kitamura, Seiji Shirakawa, Yuichiro Arimura, Xisheng Wang, Keiji Maruoka Chem Asian J. 2008 Sep 1;3(8-9):1702-14. doi: 10.1002/asia.200800107.
A very efficient, chiral phase-transfer catalyst, (S)-2 Db, was prepared by taking advantage of the combinatorial approach from the readily available (S)-1,1'-binaphthyl-2,2'-dicarboxylic acid. This catalyst exhibited high catalytic performance (0.01-0.1 mol %) in the asymmetric alkylation of N-(diphenylmethylene)glycine tert-butyl ester and N-(p-chlorophenylmethylene)alanine tert-butyl ester relative to other chiral phase-transfer catalysts in current use. This has created a general and highly practical procedure for the enantioselective synthesis of structurally diverse natural and unnatural alpha-alkyl-alpha-amino acids as well as alpha,alpha-dialkyl-alpha-amino acids. A similar simplified catalyst, (S)-2 Fb, is also applicable to the direct asymmetric aldol reaction between glycine Schiff base and aldehydes with moderate syn selectivity and high enantioselectivity.
2. Enantioselective synthesis of (R)- and (S)-alpha-alkylcysteines via phase-transfer catalytic alkylation
Taek-Soo Kim, Yeon-Ju Lee, Byeong-Seon Jeong, Hyeung-geun Park, Sang-sup Jew J Org Chem. 2006 Oct 13;71(21):8276-8. doi: 10.1021/jo061107t.
We reported efficient enantioselective synthetic methodologies for (R)-alpha-alkylcysteines and (S)-alpha-alkylcysteines. The phase-transfer catalytic alkylation of 2-phenyl-2-thiazoline-4-carboxylic acid tert-butyl ester and 2-o-biphenyl-2-thiazoline-4-carboxylic acid tert-butyl ester, in the presence of chiral catalysts (1 or 2), gave the corresponding alkylated products, which could be hydrolyzed to provide (R)-alpha-alkylcysteines (67->99% ee) and (S)-alpha-alkylcysteines (66-88% ee), respectively.
